In the field of water treatment, membrane filtration by a reverse osmosis membrane module is used as a method of obtaining domestic water, industrial water, and agricultural water from brackish water, sea water, ground water, landfill leachate, industrial wastewater and the like containing a solute such as ions and salts. The reverse osmosis membrane (RO membrane) is a membrane having a property of allowing water molecules to pass through, but does not allow impurities such as ions and salts to pass through and separates water from solutes by a pressure equal to or more than the osmotic pressure in accordance with the solute density being applied thereto. A membrane filtration system that uses such an RO membrane provides pretreatment to remove insoluble components such as turbidity, algae, and microbes contained in the intake sea water before desalination by passing the sea water through an RO membrane module. The sand filtration in which sea water is caused to permeate through a sand filled layer is commonly used for the pretreatment. However, if an attempt is made to obtain clearer pretreated water to maintain permeation performance of an RO membrane module, the sand filtration has low clarification performance and is not effective.
As an effective pretreatment method in a water treatment system, a membrane module having a microfiltration membrane (MF membrane) and/or an ultrafiltration membrane (UF membrane) is used.
In a conventional system, however, if a membrane module such as an MF membrane or UF membrane is used for pretreatment of an RO membrane module, a pressure pump to feed sea water to the MF membrane/UF membrane module (pretreatment membrane module) by applying pressure is further needed. Thus, power costs of the added pressure pump are further added to a system that uses a membrane module for pretreatment and therefore, compared with a system that uses the sand filtration for pretreatment, power costs become higher, increasing total operation costs.